Choline is an inhibitory modulator of cholinergic nerve function in guinea-pig colon

Using the novel smooth muscle-myenteric plexus (smmp) preparation from the guinea-pig colon, it has been possible to measure acetylcholine (ACh) overflow by a radiolabelling technique. The possibility that choline may be a modulator of cholinergic nerve activity in this preparation was investigated by observing its effects on electrically-evoked overflow of [3H]acetylcholine. Choline (72, 144 microM), in concentrations that did not contract the smmp preparation, caused a depression of electrically-evoked [3H]ACh overflow. This effect was unlikely to be due to actions on cholinesterase enzymes by end product inhibition. Choline also produced substantial non-muscarinic elevation of spontaneous [3H]overflow. It is concluded that inhibitory modulation of enteric cholinergic nerve activity by presynaptic muscarinic receptors may not be exclusively mediated by the actions of acetylcholine.